Gyros are known as sensors for detecting rotational angular velocities. In particular, a type of gyro which uses a vibrator is referred to as a vibration gyro and widely used for a variety of applications, such as detection of unintentional hand shaking applied to video cameras or digital still cameras, direction detection in car navigation systems, and attitude control of movable bodies such as vehicles.
Vibration gyros which have been put into practical use include a triangular-prism-shaped or quadrangular-prism-shaped vibrator to which a piezoelectric element is attached, and a column-shaped vibrator formed of a piezoelectric ceramic on which electrodes are printed (see, for example, Japanese Unexamined Patent Application Publication No. 2000-337883).
FIG. 13 shows an example of a configuration block diagram illustrating a known vibration gyro. A vibration gyro 31, composed of a vibrator 32 and piezoelectric elements 33a and 33b which are attached to the vibrator 32, is connected to vibration gyro circuitry. The vibration gyro circuitry includes an adding circuit 1, an oscillation circuit 2, a differential amplifier circuit 4, a synchronous detection circuit 5, a phase shift circuit 13, and a direct current amplifier circuit 6. The vibration gyro 31, the adding circuit 1, and the oscillation circuit 2 constitute a self-oscillation circuit 7a for causing self-oscillation of the vibration gyro 31 at a resonance frequency of bending vibration of the vibration gyro 31.
An output signal of the oscillation circuit 2 is input to the vibrator 32 and applied to the piezoelectric elements 33a and 33b through a conductive plate on the surface of the vibrator 32. An output signal of the piezoelectric element 33b and an output signal of the piezoelectric element 33a are input to the adding circuit 1 and added together. An output signal of the adding circuit 1 is input to the oscillation circuit 2 and the phase shift circuit 13.
The output signal of the piezoelectric element 33b and the output signal of the piezoelectric element 33a are also input to the differential amplifier circuit 4. The differential amplifier circuit 4 outputs a signal corresponding to a difference between the output signal of the piezoelectric element 33b and the output signal of the piezoelectric element 33a. The synchronous detection circuit 5 detects the output signal of the differential amplifier circuit 4 synchronously with a timing signal output from the phase shift circuit 13. The Direct current amplifier circuit 6 amplifies a direct current signal synchronously detected by the synchronous detection circuit 5.
The vibration circuit 31 is driven by the self-oscillation circuit 7a and performs bending vibration in an orthogonal direction with respect to the lengthwise direction thereof. When no rotational angular velocity is applied around the lengthwise central axis of the vibration gyro 31, a strain in the piezoelectric element 33b and a strain in the piezoelectric element 33a are generated in exactly the same manner. Thus, the output signal from the piezoelectric element 33b and the output signal from the piezoelectric element 33a are the same in amplitude and phase, thus resulting in an output of zero from the differential amplifier circuit 4.
When the vibration gyro 31 is applied with a rotational angular velocity around its lengthwise central axis while performing the bending vibration mentioned above, a Coriolis force is generated in a direction crossing at right angles to the lengthwise direction and the direction of the bending vibration. The generated Coriolis force causes a change in the bending vibration direction and a difference between outputs from two detection pieces (the piezoelectric element 33a and the piezoelectric element 33b). Thus, an output signal proportional to the output difference of the two detection pieces can be obtained from the differential amplifier circuit 4.
When a rotational angular velocity is applied, the piezoelectric element 33b outputs a signal in which an output signal corresponding to a drive signal supplied to the vibration gyro 31 and an output signal corresponding to the Coriolis force are superimposed. Likewise, when a rotational angular velocity is applied, the piezoelectric element 33a outputs a signal in which an output signal corresponding to the drive signal supplied to the vibration gyro 31 and an output signal corresponding to the Coriolis force are superimposed.
The output signals of the piezoelectric element 33b and the piezoelectric element 33a corresponding to the drive signal are equal in phase and magnitude, and thus cancel each other in the differential amplifier circuit 4. On the contrary, the output signals of the piezoelectric element 33b and the piezoelectric element 33a corresponding to the Coriolis force are opposite in phase and equal in magnitude. Thus, the output signal of the differential amplifier circuit 4 is proportional to the difference between the output signal of the piezoelectric element 33b and the output signal of the piezoelectric element 33a, and only a signal corresponding to the magnitude of the rotational angular velocity is output from the differential amplifier circuit 4. The drive signal for driving the vibration gyro 31 and the output signal of the adding circuit 1 are in-phase and proportional in amplitude.
A Coriolis force develops in an orthogonal direction with respect to the direction of bending vibration corresponding to a drive signal. Therefore, a signal output from the differential amplifier circuit 4 corresponding to the Coriolis force, in principle, becomes zero at the maximum amplitude point of an output signal of the adding circuit 1 which is correlated (in-phase) with the drive signal, and becomes a maximum at the zero crossing point of the output signal of the adding circuit 1. This indicates that the output signal of the differential amplifier circuit 4 and the output signal of the adding circuit 1 are phase-shifted by 90 degrees. Accordingly, the synchronous detection circuit 5 is to detect the output signal of the differential amplifier circuit 4 at a timing of an output signal of the phase shift circuit 13 which has a phase difference of 90 degrees with respect to the output signal of the adding circuit 1.